A general trend is that mines of Agromyzidae -by far the most important group of miners among the Diptera- mainly occur on herbaceous plants, while Lepidoptera miners in large majority are to be found on woody plants. Perhaps this preference among the Lepidoptera explains the observation by Klimesch (1957a) that in Austria mining Lepidoptera are hardly found above an altitude of 1300 m. A second, somewhat less sharp trend is that Diptera mines are mostly found before high summer, Lepidoptera mines after that time.
As a form rule mines preferably occur on plants in bad shape: growing in locations that are too dark, to damp or too dry, plants that are overgrown by grass or brambles, trees that are suppressed by their neighbours. Within a plant most mines occur on the older, often already damaged leaves; in herbs they must be looked for on the lower leaves. When looking for mines one therefore should not look so much on the upper, fresh leaves but fold the branches aside and inspecr the leaves in the low centre of the plant. (Striking exception: Phyllocnistis-mines are only found on the very youngest leaves.)
Plants contain a great variety of chemical compound aimed at having a deterrent effect of phytophagous insects. The concentration of these compounds vary within a species, and even within a single individual: young leaves and plant parts are more toxic than older ones. It is assumed that also plants, and parts of plants, that are under stress contain less toxins than healthy unstressed plants. Although there are exceptions to this “plant-stress hypothesis”, also in leafminers (Bultman & Faeth, 1987a) it gives a good first explanation of the distribution pattern of miners on trees and herbs (Schoonhoven, Jermy & van Loon, 1998a; Speight, Hunter & Watt, 1999a).
Seedlings can heavily be occupied, much more than ever happens in an adult plant. I can think of thwo, not completely exclusive, explanations. First, these heavily occupied, actually death-bound plants are specimens with a chemical defence mechanism that is too weak for survival, and what we see is natural selection in action. It is also possible that seedlings in general have a still weakly developed chemical defence system; in that case it is just a matter of chance of an individual escapes the fate of the plant in the picture.
Tischeria ekebladella on Oak seedling
Difference in the degree of toxicity/palatability between leaves of the same plant may also explain why often several mines, sometimes even of different species, may be found on on just one leaf, while a disproportionate number of other leaves are untouched (e.g. Ellis, Kuchlein & Meuleman-ten Broeke, 2001a and literature mentioned there). It seems that we must assume that just a few leaves are palatable among the perhaps thousands on a plant (Lawton, 1986a).
This perhaps also explains the sometimes extreme rarity of many miners, even thought their hostplants are neither rare nor small. Think of, for instance Antispila metallella, a miner of Dogwood. The host is all but rare, easily recognised, and is not growing so high that there is a part of the shrub that cannot be inspected. Yet tens of shurbs, with altogether many thousands of leaves have to be inspected in the right season before a mine is found.
A small number of species, mainly Nepticulidae are living so late in autumn thet their leaves are yellowing, or even at the point of dropping. Usually one sees around their mines a clear green zone of healthy plant tissue, that vividly contrasts with the yellow or brown autumn colours of the rest of the leaf. These so called “green islands” are caused by cytokinines, a group of plant hormones that are excreted by the larva, that suppress the aging of the surrounding leaf tissue (Giron ao). Actually, the cytokinines are not produced by the larva itself, but by endosymbiontic bacteria (Body ao; Kaiser ao). An important role is played here by the genus Wolbachia (Gutzweiler ao). By this mechanism high quality food remains available for the larve, despite the senescence of the leaf. Green islands also greatly facilitate the search for mines in fallen leaves.
Rarely probably the same phenomeon accurs in galls, like in the galls of Aspondylia sarothamni in the pods of broom.
Ectoedemia hannoverella on Black Italian Poplar
Green island around a gall of Hartigiola annulipes on Beech; the gall has just fallen off, the scar remains recognisable.
Strangely the mines of Rhamphus pulicarius (a beetle, not a moth, like the preceding example) in yellowing leaves are never surrounded by a green island.
Rhamphus pulicarius on Birch
Often the larvae in a green island, in a leaf that already has fallen, are so young that is seems questionable is they have any chance of a succesfull development. The picture below shows an example: the size of the egg indicates the minute size of this Stigmella larva.
Betula pendula, Wolfheze, 6.x.2007, in fallen leaf
Body, Kaiser, Dubreuil, Casas & Giron (2013a), Bultman & Faeth (1987a), Connor & Taverner (1997a), Ellis, Kuchlein & Meuleman-ten Broeke (2001a). Giron, Kaiser, Imbault & Casal (2007a), Gutzwiller (2011a), Gutzwiller, Dedeine, Kaiser, Giron & Lopez-Vaamonde (2015a), Kaiser, Huguet, Casas, Common & Giron (2010a), Klimesch (1957a), Lawton (1986a), Schoonhoven, Jermy & van Loon, (1998a), Speight, Hunter & Watt (1999a).